24 September 2012

Drought and Climate Change

The amount of nonsense in public debate on extreme events and climate change remains at a high level. This is great news for me because it provides plenty of opportunities to discuss what the actual science says and how we think we know what we know. As I have long argued, accurate representations of the state of science of extremes is far more important as a matter of scientific integrity than to the hyper-politiczed debate over climate change. Put another way, it is highly unlikely that misrepresentations of the state of science will do much to move action on energy policies, but they could damage the integrity of leading institutions of science.

This is a short post about drought, which simply summarizes the bottom-line conclusions of two of the most recent major scientific assessments of extreme events and climate change, one by the US government, released in 2008 under the Bush administration (PDF, and then reaffirmed in the CCSP Unified Synthesis under the Obama Administration, here) and the second from the IPCC.

First, from the US government's assessment of extreme events in the US, here is what it concluded about drought (here in PDF, at p. 42):

The most widespread and severe drought conditions occurred in the 1930s and 1950s (Andreadis et al., 2005). The early 2000s were also characterized by severe droughts in some areas, notably in the western United States. When averaged across the entire United States (Figure 2.6), there is no clear tendency for a trend based on the PDSI. Similarly, long-term trends (1925-2003) of hydrologic droughts based on model derived soil moisture and runoff show that droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the U. S. over the last century (Andreadis and Lettenmaier, 2006). The main exception is the Southwest and parts of the interior of the West, where increased temperature has led to rising drought trends (Groisman et al., 2004; Andreadis and Lettenmaier, 2006). The trends averaged over all of North America since 1950 (Figure 2.6) are similar to U.S. trends for the same period, indicating no overall trend.

Got that? Over the climate time scales "droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the U. S. over the last century." At the top of this post is Figure 2.6 from that report, showing drought incidence in the US (red) and North America (blue) from 1900.

The IPCC in 2012 conducted a survey of drought globally, and concluded with the following (here in PDF, at p. 171):

There is not enough evidence at present to suggest high confidence in observed trends in dryness due to lack of direct observations, some geographical inconsistencies in the trends, and some dependencies of inferred trends on the index choice. There is medium confidence that since the 1950s some regions of the world have experienced more intense and longer droughts (e.g., southern Europe, west Africa) but also opposite trends exist in other regions (e.g., central North America, northwestern Australia).

Got that? Some places have become dryer, others wetter, and not much confidence in asserting the presence of any trends at the global scale.

Now it is of course true that the recent assessments of the US government and IPCC are not the last words on these subjects. They represents attempts by governments to have scientists systematically arbitrate questions that can resolved empirically, such as "has drought increased or decreased?"

Anyone who shows up in public debate (or the scientific literature) with an alternative view (e.g., droughts have become worse in the US or there is more certainty at the global level) had better come with some strong evidence. The need for strong evidence comes not simply from the authority of these assessments, but because they represent a condensation of a large amount of literature. To overturn the conventional wisdom expressed in the assessments requires overturning the arguments found in the literature on which the assessment is based. The same logic goes for people who would challenge other findings from such assessments, such as in making a claim that the globe is not warming.

The findings of assessments are not however received truth. As we have seen, those leading and participating in such assessments can err egregiously -- where an error means a failure to accurately reflect in the assessment the underlying scientific literature. Here overturning the assessment is relatively easy, as it does not require over turning the literature, but simply representing it accurately. This is why the IPCC SREX came to different conclusions on extremes than did AR4.

Unfortunately, there are no shortcuts here to reliable knowledge - just because the IPCC erred on one subject does not mean that it necessarily erred on others. Each knowledge claim must be evaluated on its merits. Shortfalls in an assessment process may cause you (or me) to lose confidence in the integrity of that process, but it does not eliminate the need to evaluate knowledge claims on their merits using the methods of science.

The good news is that for extreme events and climate change there is a well-developed scientific literature on this topic, and it does not always jibe with what you might read in the news or hear from experts. Given the politicization of the climate debate, and the penetration of that politicization into academia, I don't expect this to change anytime soon, if ever.